High modulus plastics which also exhibit superior toughness, possess good impact resistance, allowing them to accept sudden blows and shocks without being fractured, or suffering substantial physical damage. Such materials, frequently referred to as engineering, or high-impact plastics, have broad applications in the formation of articles that are expected to be subjected to abrupt, extreme stress during use.
In the past, a variety of "toughened" plastics have been prepared and used for high-stress applications. These include materials such as high impact polystyrene, "ABS", and terpolymers of acrylonitrile-butadiene-styrene. In such polymers, soft, rubbery segments are polymerized in block sequences, or grafted onto polymer backbone chains. Typically, the rubbery domains are microphases separated from the glassy matrix constituting the balance of the material. When such polymers are subjected to external forces, energy absorbing crazes develop at the boundaries of the rubbery domains, making possible the toughness exhibited by the materials.
Polymer mixtures with improved impact characteristics have also been prepared. For example, acrylonitrile-styrene co-polymers have been incorporated with polycarbonates, the former being brittle polymers, and the latter ductile substances. Polycarbonates have also been incorporated with polymethylmethacrylate, another brittle polymer. In both instances, improvements in toughness have been attributed to the development of stress-created crazing at the peripheries of the brittle domains, the process of craze formation absorbing stress energy, and preventing significant damage to the material. While such absorption is accomplished in a relatively non-destructive fashion, clearly, there is a finite limit to the amount of stress that can be accommodated by craze formation.
A different approach, the one contemplated by the invention disclosed herein, involves the preparation of discrete polymeric molecular entities as a single composition, which however, entails separate solid phases with interconnected spinodal structure in the area of the phase boundaries. Such interconnected structure, sometimes termed modulated structure, has been discovered to exert a profound affect on the mechanical performance of the polymers exhibiting such morphology, particularly the toughness of such polymers.